Use default node attributes for download_url, filename and pid_file

As discussed in #245, this is @jakekdodd's solution. Will extensively test this in the coming days.

Systems that upgraded from an earlier version to the #242 patch should still have the ```download_url```and ```filename``` node parameters cached in the Chef Server.
New installations with that cookbook will have those attributes 'nil' or missing entirely. This patch should restore functionality.